Capacitive discharge ignition exciter circuits in wide spread use for gas turbine engines are of the types of variations of the types described in U.S. Pat. No. 3,619,638 "Pulse Generating Apparatus" by E. M. Phinney, issued Nov. 9, 1971 and in U.S. Pat. No. 3,531,738 "Continuous Duty Ignition System" by K. H. Thakore, issued Sept. 29, 1970. In each of the referenced exciters a control spark gap, not to be confused with the igniter plug spark gap, is used to initiate discharge of a storage capacitor into a conditioning circuit which transforms the discharge current from the storage capacitor in a high voltage, high energy pulse for supply to the engine igniter plug. The engine igniter plug is analogous to the spark plug of an Otto cycle engine in that is functions to provide a spark discharge to initiate combustion of the fuel mixture in the engine combustion chamber. Ignition systems for turbine engine ignition systems supply spark continuously to the engine instead of supplying spark in timed relationship to the engine cycle and in that the energy and instantaneous power requirements for a turbine engine system are much higher than those of an Otto cycle engine system.
The lack of a requirement to supply timed spark in a turbine ignition system leads to certain simplifications in the exciter circuit, among which is the use of a control spark gap to initiate discharge of the storage capacitor. The control spark gap normally comprises a pair of substantial electrodes enclosed in a gas tight housing together with some type of radioactive emitter of beta particles. The beta particles ionize the gas within the housing to promote discharge through the gap at more uniform voltage levels.
Control spark gaps create service problems in turbine ignition systems. Spark erosion of the electrodes necessitates more frequent replacement of the spark gaps than is desirable and the radioactivity of material used in the gaps demands that special procedures be used in the manufacture, storage, handling and disposition of the gaps. For these reasons, consideration has been given in the past to replacement of the control spark gap with an alternative form of switching device, such as a silicon controlled rectifier (SCR).
SCRs have been used as capacitor switching devices in prior capacitive discharge ignition systems designed for use in Otto cycle engines. Two examples of such ignition systems are seen in U.S. Pat. No. 4,232,646 for "Ignition System for Internal Combustion Engines with a Magneto Generator", issued Nov. 11, 1980 and in U.S. Pat. No. 3,605,714 for "contactless Ignition System", issued Sept. 20, 1971. In each of these patents a single SCR is connected in series with the storage capacitor and the primary of an ignition transformer. At the appropriate time in the engine cycle a trigger pulse is applied to the gate electrode of the SCR, switching it into conduction and discharging the storage capacitor through the ignition transformer to generate an engine spark.
The Otto cycle engine circuits use low voltage SCRs to switch the low voltage side of an ignition coil to develop a current limited, low instantaneous power, high voltage pulse for the spark plug. Such circuits were not designed for, nor can they deliver the required instantaneous power to fire a turbine engine igniter plug effectively.
The arrangements of the referenced systems do not provide failsafe operation in the event of the occurrence of an anode-cathode short circuit in the SCR and such short circuits are the most common modes of failure in such devices.
It is an object of the present invention to provide an ignition system for a gas turbine engine which does not require the use of control spark gaps.
It is a further object of the invention to provide an ignition system of the capacitive discharge type for a gas turbine engine in with SCRs are used as capacitor switching devices.
It is still another object of the invention to provide a capacitive discharge ignition system in which SCRs are used as capacitor switching devices and in which the SCRs are connected in circuit in such manner as to provide for continued operation of the system in the event of a short circuit failure of one or more, but less than all, of the SCRs.